The submillimeter frequency shifter is not restricted by the usual limitations of small aperture, low efficiency, and restricted frequencies of operation common to laser frequency shifters currently under development or in use for such applications as frequency chirping for laser radar, laser isotope enrichment, fluorescence spectroscopy, and optical pulse compressors. Due to small linear and angular apertures and very high absorption at the longer wavelengths, prior art frequency shifters are unsuitable for operation in the submillimeter wave region of the spectrum near 1 mm. These prior art frequency shifters depend upon the bulk properties of solid materials whereas the present frequency shifter utilizes optical properties of narrow absorption lines in gases that are due to the molecular structures of the gases.